if you are in the folder where the archived package sits, and open a console from there, you will type something like this:
installpkg packagename-1.2.3-noarch-slack11.tgz
In other words, you use the FULL name of the package, and it must be a Slackware-built package in order to use 'installpkg' to install it.
If it isn't a Slack-package, but a package of sourcecode for a non-specific distro, then you need to extract the package, and follow any instructions inside it in README file(s) to configure, compile, and install the application.
Does this help?